Drop support for Java 7

[flowersinthesand]

I have thought that we have to drop support for Java 7 in a new major release (2.0.0) but now think we can do that in a minor release (1.2 or 1.3) since 1.1 will be full-featured and no one seems to use Cettia with Java 7. As a bonus, I found that Jetty dropped support for Java 7 as of a minor release, 9.3.

• Update .travis.yml.
• Replace anonymous type with lambda expression.
• Replace lambda expression with method reference.

[flowersinthesand]

• Use JDK's Base64

[flowersinthesand]

• Annotate functional interfaces with @FunctionalInterface